FIG. 2 shows a cross-section of a vacuum pump 50 known hereto which comprises a pumping arrangement driven by a single shaft. The arrangement shown comprises a turbomolecular pumping mechanism 52 and a Holweck pumping mechanism 54, the latter of which is a molecular drag pumping mechanism. The rotors 58 and 59 of the turbomolecular pumping mechanism and the Holweck pumping mechanism, respectively, are arranged to be driven by shaft 56 so that when the shaft is rotated by a motor 60 the shaft drives the pumping arrangement 52, 54. The shaft 56 is supported by a bearing arrangement comprising two bearings which may be positioned either at respective ends of the shaft as shown or alternatively intermediate the ends. In FIG. 2, a rolling bearing 64 supports a first portion of the shaft 56 and a magnetic bearing 62 supports a second portion of the shaft 56. A second rolling bearing may be used as an alternative to the magnetic bearing 62. When magnetic bearings are used it may also be desirable to incorporate a back-up bearing as well known in the art. As discussed in more detail below in relation to FIG. 3, the rolling bearing 64 is provided between the second end portion of the shaft 56 and a housing portion 66 of the pump 50.
With such a pump, it is desirable to allow rolling bearing 64 some movement in the radial direction (radial compliance) but to prevent movement in the axial direction. Any axial movement can lead to clashing between the rotor blades of the turbomolecular pumping mechanism and the stator resulting in pump failure. It is advantageous to allow the radial bearing some radial movement in order to reduce the transfer of vibration from the pump rotor to the pump housing, caused by residual imbalance.
The prior art bearing arrangement will be explained with reference to FIG. 3 which shows an enlarged view of the rolling bearing 64. The rolling bearing comprises an inner race 68 fixed relative to shaft 56, an outer race 70, and a plurality of rolling elements 72, supported by a cage 73, for allowing relative rotation of the inner race and the outer race. The rolling bearing 64 is lubricated to reduce wear on its elements and shield elements 74 are provided to resist seepage of lubricant out of the rolling bearing. The shield may be a separate component, held in place by a spring clip, or other fastener, or alternatively may be an integral part of the bearing outer ring. A radial damping ring 75 is positioned radially between the outer race 70 and the housing portion 66 for damping radial movement of the outer race 70. An axial damping ring 76 is provided between an end face of the outer race 70 and the housing portion 66 which resists axial movement of the outer race but allows radial movement thereof. However, the axial damping ring 76 does not adequately resist axial movement of the outer race because it is, to some extent, compressible in the axial direction and suffers from creep (or compression set) which makes the problem worse over time.
Furthermore, even though a lubricant is used in the rolling bearing 64, due to the potentially high rotation speeds of the pumping arrangement, the rolling bearing increases in temperature during operation. Such an increase in temperature leads to rapid failure of the rolling bearing unless heat can be dissipated from the rolling bearing. A further problem with the prior art arrangement is that the axial damping ring 76 is made from an elastomer which has a low thermal conductivity and is resistant to the passage of heat from the outer race to the housing portion. The housing portion is typically made of Aluminum alloys and can be maintained at a relatively low temperature since such materials have a thermal conductivity in the region of 150 W/mK.
It is desirable to provide an improved vacuum pump.